This invention relates to improved well instruments which are protected against high formation temperatures.
Many types of instruments which are lowered into wells in order to sense a desired condition or perform a a desired operation include components which can be damaged or otherwise adversely affected by the often very high downhole temperatures encountered in wells. For example, surveying instruments and directional drilling steering tools may contain sensors responsive to inclination and direction which can not operate satisfactorily at extremely high temperatures. Because of the very limited horizontal cross-sectional area of a well bore, it is extremely difficult to effectively protect the interior parts of such an instrument from the high temperature of the downhole environment, and consequently it is often necessary to limit drastically the amount of time an instrument is allowed to remain in the well performing the sensing or other operation, and then quickly pull the instrument out of the hole before damage or other adverse effect can occur. The problem is compounded by the fact that the instrument in operation may itself develop additional heat which will tend to raise the temperature of the instrument and related equipment even if it is isolated relatively effectively from the temperature of the surrounding formation.
U.S. Pat. Nos. 3,265,893 issued Aug. 9, 1966 to T. A. Rabson et al. and 3,859,523 issued Jan. 7, 1975 to Wilson et al. show well logging instruments which are enclosed within dewar flasks, that is, containers having spaced walls with a vacuum therebetween, to heat insulate the interior of the container from its exterior. These dewar structures in the two mentioned patents also contain heat sinks for absorbing heat developed in the interior of the dewars. U.S. Pat. No. 3,859,523 specifies that the heat sink may be formed of a fusible material acting to absorb heat at a constant temperature during melting.
U.S. Pat. No. 3,435,629 issued Apr. 1, 1969 to J. K. Hallenburg shows a device including downhole electrical apparatus contained within a dewar which also contains a body of water from which vapor is withdrawn by a pumping system to the exterior of the dewar to maintain a desired vapor pressure and thus attain a desired water and instrument temperature within the dewar, and with the water being forced to condense at high pressure at the outside of the dewar.